refactor: replace config.yml with .env for all configuration

All settings (auth credentials, scanner ranges, status_checker interval)
now live in a single .env file via pydantic-settings. config.yml and
config.yml.example are deleted.

- Settings: add auth_username, auth_password_hash, scanner_ranges,
  status_checker_interval; add load_overrides()/save_overrides() for
  persisting runtime changes to data/scan_config.json
- auth.py: read credentials directly from settings
- scan.py: read ranges/interval from settings; write-back via save_overrides()
- scheduler.py: read interval directly from settings
- main.py: call settings.load_overrides() at startup
- docker-compose.yml: remove config.yml volume mount, add new env vars
- conftest.py: set settings fields directly instead of writing a temp config.yml

backend/tests/test_scan.py

@@ -49,8 +49,9 @@ async def test_trigger_scan_requires_auth(client: AsyncClient):
 async def test_trigger_scan_creates_run(client: AsyncClient, headers):
     with (
         patch("app.api.routes.scan._background_scan", new_callable=AsyncMock),
-        patch("app.api.routes.scan._load_ranges", return_value=["192.168.1.0/24"]),
+        patch("app.api.routes.scan.settings") as mock_settings,
     ):
+        mock_settings.scanner_ranges = ["192.168.1.0/24"]
         res = await client.post("/api/v1/scan/trigger", headers=headers)
     assert res.status_code == 200
     data = res.json()